1. Field of the Invention
This invention relates to signal receivers. Particularly, this invention relates to double sampling receivers for optical or electrical signaling.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Integrated circuit scaling has enabled a huge growth in processing capability, which necessitates a corresponding increase in inter-chip communication bandwidth. This trend is expected to continue, requiring both an increase in the per-pin data rate and the I/O number. Unfortunately, the bandwidth of the electrical channels and the number of pins per chip do not follow the same trend. As data rates scale to meet increasing bandwidth requirements, the shortcomings of copper channels are becoming more severe. While I/O circuit performance favors from technology scaling, the bandwidth of electrical channels does not scale with the same trend. In particular, as data rate increases, they pose excessive frequency-dependent loss, which results in significant intersymbol interference (ISI) [1]-[3]. In order to continue scaling data rates, equalization techniques can be employed to compensate for the ISI. However, the power and area overhead associated with equalization make it difficult to achieve target bandwidth with a realistic power budget. As a result, rather than being technology-limited, current high-speed I/O link designs are becoming channel- and power-limited.
A promising solution to the I/O bandwidth problem is the use of optical interchip communication links. The negligible frequency-dependent loss of optical channels provides the potential for optical link designs to fully utilize increased data rates provided through CMOS technology scaling without excessive equalization complexity. Optics also allows very high information density through wavelength-division multiplexing (WDM). Hybrid integration of optical devices with electronics has been demonstrated to achieve high performance [4]-[9], and recent advances in silicon photonics have led to fully integrated optical signaling [10]-[11]. These approaches pave the way to massively parallel optical communications. In order for optical interconnects to become viable alternatives to established electrical links, they must be low-cost and have competitive energy and area-efficiency metrics. Dense arrays of optical detectors require very low-power, sensitive, and compact optical receiver circuits. Existing designs for the input receiver, such as TIA, require large power consumption to achieve high bandwidth and low noise and can occupy large area due to bandwidth enhancement inductors. Moreover, these analog circuits require extensive engineer efforts to migrate and scale to future technologies.
With the increasing bandwidth requirements of computing systems and limitations on power consumption, optical signaling for chip-to-chip interconnects has gained a lot of interest. Dense arrays of optical detectors require very low-power, sensitive, and compact optical receiver circuits. Existing designs for the input receiver, such as the transimpedance amplifier (TIA), require large power consumption to achieve high bandwidth and low noise, and can occupy a large area due to bandwidth enhancement inductors. In most optical receivers, the photodiode current is converted to a voltage signal. A simple resistor can perform the I-V conversion if the resulting RC time constant is in the order of the bit interval (Tb). However, for a given photodiode capacitance and target signal-to-noise ratio (SNR), the RC limits the bandwidth and hence the data rate. To avoid this problem, TIAs which are highly analog, power hungry are commonly employed, and do not scale well with the given technology. One alternative is to integrate the front-end to eliminate the need for resistance and break the bandwidth trade-off. However, this technique suffers from voltage headroom limitations, and requires short-length DC-balanced inputs.
In view of the foregoing, there is a need in the art for improved apparatuses and methods for optical receivers. There is particularly a need for such apparatuses and methods to operate at less than full bandwidth. Furthermore, there is a need for such apparatuses and methods to operate at very low power. These and other needs are met by embodiments of the present invention as detailed hereafter.